Chromium plating



?atented bee. 5, 1939" at: OMIUM PLATIN'G Frank H. Beall, Maplewood, N. J.

No Drawing.

8 Claims.

The invention relates to the electrodeposltion of chromium and is more particularly concerned with a process employing a novel chromium plating solution and special preparation of the metal surfaces preliminary to the plating operation per se.

The object of the invention is to provide a process of chromium plating which is an improvement on present practices particularly in being less sensitive to variations in the factors involved in chromium plating and in the quality of the chromium plate produced.

A common practice in chromium plating is to use a plating bath composed essentially of a water solution of chromic acid (CrOs) and a sulplate or sulphuric acid, the content of chromic acid varying, in general, from 150 to 450 grams per liter and the sulphate, estimated as S04, be-

$04 to C103 is from 1 to 50 to 1 to 250, i. e. the S04 amounts to from 2% to 0.4% of the C103 content. But in actual practice the ratio of S04 to CrOa is between'0.66% and 1.15%. Seemingly slight departures from the prescribed conditions of the so. to CrOs ratio, the concentf'ation of the CrOa, and the temperature result in detion I will refer specifically to salts of sodium,

such salts being more readily available commercially,but it should be understood that it is not intended to confine the invention to the use of sodium salts, as soluble salts of potassium, ammonium, zinc, and other bases are suitable. The essential characteristic of the additional metal is that it forms with the chromic acid a soluble bichromate, the metal of which does not deposit or the salt'precipitate or form a colloidal solution. Blchromates of limited solubility can be mixed with those of greater solubility.

There are several ways in which the plating. bath can be made up. A preferred method, disfiled August 21, 1933, of which the present application is in part a continuation, comprisesmixing the following:

Chromic acid (CrOs) pounds 60 The chromic acid and alumina hydrate are 90 ing preferably about 1% of the CrOa content.

The extreme theoretical range for the ratio of closed in my prior application, Ser. No. 686,069,.

Application October 15, 1936, Serial No. 105,799

mixed first and then the sodium carbonate and water added in alternating increments, the final product being about 125 lbs. of a yellowish powder. (The final content of $04 in this mixture as shown byanalyses is about 21-22% of the. powder or about .27% of the total CrOa.) The plating bath is made up by dissolving the mixture in water, about three pounds to the gallon being preferred.

The composition may be obtained by direct admixture of CrO: and sodium bichromate or equivalent chromate, the choice of reagents to convert the 'CrOs into bichromates or to supply the chromate or bichromate being merely a question of relative cost of materials of the necessary purity.

The alumina hydrate employed is a commercial product containing a fairly definite proportion, about 12% of $03. The alumina hydrate, being a bulky material, facilitates uniform mixing. The commercial CrOa also contains sulphate as an impurity as may also the NazCOs. But in the 'actual practice of my invention, the total sulphate .is preferably around or below 0.1 oz. per gallon of bath as compared with three or four times as much in the conventional high acid bath. The above mixture will give about 0.1 oz. per gallon of plating solution containing 3 lbs. of the mixture. Of course, other sulphate carriers than the alumina hydrate can be used, such as sodium sulphate in proper proportions, the essential ingredients being free CrOa, the salt of C10: and

the catalytic acid radical preferably S04. V The exact proportion of the hexavalent chromium present in the form of the salt (considering the salt to be bichromate) is not critical, 70% being generally suflicient. Solutions in which 80% is in the combined form have a slightly higher throwing power but lower conductivity. Much above 80% results in difficulties in obtaining a good plate. Operating conditions will 1 therefore affectthe ratio selected, for example, the relative desirability of lower resistance and of higher throwing power. In general 70% is recommended and less than 60% is not preferred. The preferred method outlined above gives neu tralization of chromic acid to bichromate of approximately 70%. I

Variations in concentration of the bath content are also possible. These variations are limited on the upper-side by solubility of the compound and on the lower side by a darkening or lack of uniformity of the deposit. I have, therefore, found'it desirable to maintain the mixture at not less than- 30 oz. per gallon. The concentration of free CrOz, however, should be maintained at not less than 5 oz. per gallon even when lower concentrations of-total chromium arefused. I have found that variations in the sulphate concentration in this plating solution over a very wide range when operated as hereinafter described give results that produce a flat bright plate characteristic curve. This curve from a concentration of about .25 ounce of $04 per gallon on down to the lower limit practically possible by operating with commercial CrOz shows a substantially uniform bright plate range. The range, however, is somewhat wider at the lower S04 concentrations. It was for this reason and also for aminimum etching action of the solution that I prefer the .1 oz. $04 per gallon concentration, which concentration can be conveniently maintained with average commercial 0103 even when taking into account the consumption factor which tends to build up 304 from this impurity in the CrOs consumed. with much above the .25 oz. $04 per gallon, however, the bright plate range rapidly narrows, as reduced chromium oxides are formed at a faster rate than they are dissolved by the solution. This difficulty is of course greater at lower mixture concentrations, which is the reason for choosing the approximately 48' oz. per gallon-solution concentration as the preferred one. Whilehigher solution concentrations possess certain advantages, the

drag out factor also increases, causing increased cost.

I have found that, by varying the proportions of chromic'acid salts, the ratio. of hexavalent chromium to $04 can be widely varied and plating solutions obtained which are very low in 80.; content. For example, a solution of a high grade of chromic acid containing 0.085% $04, which without addition of more sulphate will give no indication of plating at normal current densi- :ties, can be made to plate upon neutralizing a substantial'part of the chroinic acid with sulphate-free NaOH. In such solution the ratio of hexavalent chromium to S04 is about 1180 to 1.

Good results with such solution have been obtained when the C103 was neutralized to about 80% (estimating the neutralized CrOa to be in the form of [sodium bichromate) leaving 2 of the CIO: as free-acid, with the total original 0103 content about 40 oz. to the gallon. Consistent plating has been obtained with the neutralization from 75% to 85% but if as much as of the CIOs is neutralized no plating at all is obtained. It will be noted that, with 75% neu- 4 tralization, the S04 is 0.34% of the free or unneutralizedCrOa, and at 90% neutralization the S04 is 0.85% of the free CrOa.

It is my belief-that the salts thus introduced serve to dissolve or to maintain in solution the salts or oxides of reduced chromium which otherwise. deposit along with metallic chromium andcause iridescence, splotches, or dark deposits. The bath differs, however, from all former'chromium plating baths, of which I am aware, in the following particulars: first, the total sulphate radical relativeto the total CrO: radical present is lower than is practicable in any known commercial bath. I I and reduces the oxidizing and etching effect on the metal surfaces to be plated); second, the sul- -phate radical present has no definite relation or to the 7 either to the neutralized Cl'Os present unneutralized radical present.

The maintenance of the solution is very simple.

The proportions are much less criticalth'an in the high acid baths in presentuse and'commercialbaths have been controlled over long periods by tests'of the Baumgravity only, by merely adding material of the original composition plus and zinc-base alloys.

(This-feature lowers the acidity a percentage of CrOs to compensate for the consumption of the cros. Frequent chemical analyses are unnecessary.

In plating-with my low acid bath it has been found that the deposits are .whiter, i. e., they are free from the bluish cast of conventional chromium plate. Also the deposit is less brittle and there is a practically complete absence ofI 'lin,

cracks commonly experienced with higher etching the. zinc at room temperatures with a reasonably short time of immersion. The etching action of these solutions, higher in $04 per gallon, is easily ascertained by flashing a coating of zinc upon a strip of'brass and then dipping it in the solution. If the etching action is detrimental the brass color will rapidly reappear.

The following is given as 'aspecific example of commercial practice in the plating of zinc base die castings in my solution, or other metals which respond in the same way to the plating solution, such as lead, pewter and silver. The work is first cleaned in a solution of .NaOH "ounces per gallon" 8 NaCN do to The work is made ,a cathode and chromium I tank voltage used. It is then rinsed well and put in the plating tank. i

For metals of this type the solution temperature is preferably kept-be1ow 80 F. but no strict control of temperature is necessary, and the usual room temperature is satisfactory. Tanks without heating or cooling equipment, have operated well as low as 55 F. in the winter and up to 85 in the summer. The current density should be about 100.amperes per square foot on metals of this type.

The work comes from the tank with a grey smut having a slight indigo blue cast. I This'smut colors oif almost as readily as the smut on a good nickel plate. In practical operation the current is pushed up to the maximum limit at, which the work, after a three minute plate, still shows the indigo blue cast; A further increase will result in a more greyish color; which is harder to color" up. At the lower temperature the smut is often changed to a slight brdwnish color by the slower rate of dissolving of the CraO: and other reduced compounds. This, however, has not proved to be detrimental either to the clearness' of the plate or to the readiness with which it colors up. Allowing the piece to remain in the solution fora short time after disconnecting will dissolve these products to give the color control. I

. The characteristics of my bath not only enable it to plate chromium directly upon articles, such as zinc-base die castings, or articles of lead,

pewter .or silver; that cannot be satisfactorily plated in the usual bath, but also enables it to plate successfully on nickel, copper, 'brass, etc., after the work has been subjected to preliminary cleaning and etching operations which are adapted seconds in this solution using chromium tank;

to. the particular metal surface to be plated.

The bright plate range is not controlled by the chemical composition of the solution and temperature alone, but is very largely dependent at the lower SO; concentrations on the preliminary treatment of the metal surface.

The invention accordingly contemplates the use of certain preliminary treatments adapted to and in combination with the subsequent plating in my low acid bath. By these treatments I apparently invisibly etch or roughen the metal surfaces so that the proper bond is produced in my plating solution to which the work is subsequently subjected.

In determining the ingredients of the etch solution, the products of their reaction should be soluble in the etch solution itself.

For nickel I use a mixture of an oxidizing reagent such as Cr03 and an acid such as HCl. A properly etched surface, free from insoluble oxidation products, is obtained for the above preferred" plating bath by the following solution:

CrOa ounces per gallon 2 NaCl do .04 to .06

The concentration of the chlorine radical is I the determining factor; and the CTO: has to be made sufiiciently concentrated to act as a dilute acid wash to neutralize any alkali which may carry over from the usual previous cleaning operation.

An etch solution involving an increased amount of acid radical and a bichromate, such as sodium or ammonium bichromate, results in a wider plating range for the truly bright plate. For example, such an etch solution may be as follows:

Sodium. bichromate ounces per gallon 20 @IOs don 2 NaCl do .04 H2804 do .1

alent amounts of soda ash and chromic acid maybe added. As commercial soda ash contains sodium chloride as an impurity, the employment of this combination would operate to add additional sodium chloride.

The procedure for the plating of a nickel surface is:

(a) Clean work in cleaning solution and rinse with water.

(1:) Etch for'about ten seconds in nickel etch solution at room temperature. Time is not critical. Then rinse in water.

(c) Enter work in the chromium tank, making sure that. it is well wet with the plating solution before making" contact with the cathode bar.

For plating copper and brass, the proper etch can be obtained by using the correct proportionsof cyanide in the cleaning solution.

An example of such an etch solution is as follows:

NaOH ounces per gallon 8 NaCN dn 1 /2 Copper cyanide do This solution should be used at room temperature. The procedure is;

(a) Clean electrically at cathode for 5 to voltage, and rinse in water.

(b) Rinse in a 1 to 2% H01 solution to remove (d) Enter the work in chromium tank and' plate.

The plating solution of my invention possesses a good bright plate range on copper, brass and nickel, even at temperatures of 110 F. but the best general range on these metals is 80-90 F.

While I have outlined different etching solutions for the diflerent metals which I have found preferable for my preferred plating solution, I must make clear that these exact proportions for the etching solutions will not be the optimum for the entire range of variation in plating solutions having sulphate radical concentrations below approximately .65 percent, but would have to be varied according to the departures from the preferred plating solutions and directions given above. It should be understood that it is not intended to confine the invention to the use of the preferred nickel etching solution described above. Since I have found that the salts of both hydrochloric and sulphuric acid react as acids through ionization in the presence of weak acids, such as chromic acid, it is obvious that hydrochloric or sulphuric or other strong acids such as hydrofluoric acid can be used in combination with an oxidizing reagent, such as chromic acid to make suitable nickel etching solutions. In the substitution of other strong acids for hydrochloric acid, it should be observed that the most suitable proportions of acid will not depend upon 1 the comparison with hydrochloric acid upon the basis of hydrogen ion concentration. Different acids have different etching characteristics in the presence of an oxidizing reagent, such as chromic acid. The proper proportions for other strong acids will easily be found. When the action of the etching bath is too strong, unplated areas form 0.1 the work in the plating operation; when too weak, the throwing poweris poor and the resulting plate hazy.

It should also be understood that it is not intended to confine the invention to the use of the preferred copper and brass etching solution since the concentration of the cyanide (the etching medium) in the alkali cleaning solution and the concentration of the hydrochloric acid in ,be used equally well.

I have found that it is possible to etch copper and brass in a properly proportioned oxidizing solution similar to, though considerably weaker than that used for etching nickel; but that the alkali cyanide solutions are preferable for these metals due to the uncertainty of the solubility of the oxides of copper in solutions of this type.

I have found also that even the above preferred plating solution when used for plating copper .operation.

and brass have'some, though an insuflicient etching action, on the surface to be plated. This mild etching action of the plating solution on the surface of the metal to be plated should be taken into account in determining the degree of etching action desired in the preliminary etching .The etching action of the preferred plating A solution on nickel is practically negligible; but

it higher ratios of $04 to total CrOa are used, .the etching action is not negligible and must be taken into account and allowed for in determining the degree of etching action desired in the preliminary etching operation.

While I'have described the use of plating baths containing bichromate andflow in S04 concentration with my etching baths, it should be understood that the invention is not to be confined .to the use of my etching baths with such plating solutions, for myetching baths can be usefully employed with plating solutions containing little or no bichromate (representing neutralizations of Cl'Oa from 60 per cent down to zero per cent) but containing $04 in lower concentrations than .65 per cent. Although such plating solutions havea greater etching action than similar baths containing substantial amounts of bichromate as described above,-they are still useful for plating with my etching solutions. If the S04 concentration of such baths is reduced, the

' etching action-is correspondingly reduced until will retain'its luster for a long period.

'50 (2) -A much more adherent and lasting plate,

it practically ceases at the lowest concentrations.

Higher S04 concentrations though still below .65 per cent result in an etching, action which must be taken into account and allowed for in determining the degree'of etching action desired in the preliminary etching operation. This etching action of the higher S04 concentrations (though still below .65 per cent) is little affected by small amounts of bichromate. But if the proportion of bichromate is increased, the efiect is to diminish'the etching action until at the preferred percentages of neutralization, the etching action is-reduced to minimum proportions.

The advantages of my process over the present high-acid method are as follows:

( 1) Ability to plate metals of the type of zinc die castings direct with an adherent plate that free from line cracks, on'brass and copper.

(3) On nickel the ease with which a true bright plate may be obtained even with irregular hand operation in the plating bath.

(4) No changes take place inthe etching solution to upset the equilibrium so thatcontrol is greatly simplified.

(5) The plating solution is much less sensitive to the proportions of the chemical components present; e. g., Na, CrOa, and S04, so that its maintenance is much less critical. There is no necessity to maintain a substantially constant ratio of S04 to either the totalCrOa present or to the free CrOa' .present.

(6) The temperature may vary over a considerable range without afiecting the results.

,While I have referred above to thesulphate content, it should be understood that the invention is iot limited tosuch acid radicaladditional agent or .catalyst. Other such addition agents known in the art, fluorides, fluosilicates,'etc., may be employedin reduced amounts in accordance with the present invention. The impurity present in the CrOa and other materials, however, must be taken into account and corresponding reduction being 30 to 50 oz. per gallon.

made in the additions of such catalysts. As sulphate is already present in the commercial raw material in amounts. that roughly correspond with that used, the use of such equivalent known catalysts would largely depend, on obtaining a raw material of greater purity than presently commercially available. I I claim;

1. A low-etching chromium plating bath for plating clear, adherent chromium comprising,

a water solution of chromic acid in part free and .in part combined as an alkali salt, the free acid constituting 20 to 40% of the total and not less than approximately 5 oz. per gallon, and sulphate to the amount of not over approximately .4% of the total chromic acid, the total chromic acid 2. A chromium platingbath for plating clear, adherentv chromium comprising, a water solution of chromic acid in part free and in part combined as a metal salt on chromic acid, the metal of which does not deposit or the salt precipitate or form a colloidal solution, the total chromic acid being about at least 30 oz. per gallon and the free-chromic acid constituting 20% to about 40% of the' total, and a catalyst consisting of at least one of the acid radicals of the following class, sulphates, fluorides, and fluoscilicates,-to

lic surface composed preponderantly of .a metal of the class consisting of copper,. zinc, lead and silver, comprising immersing the article in an aqueous plating bath containing free chromic acid, a soluble bichromate and a sulphate catalyst promoting deposition of chromium, the total chromic acid-being about 30 to 50 oz. per gallon and the free chromic acid constituting 20% to about 4.0% of the total, said sulphate catalyst .-,being present to the amount of not over approximately .4% of the total chromic acid, and supplying plating current to said article.

5. A process of producing an adherent coating of clear, adherent chromium upon an article having a metallic surface composed preponderantly {of a metal of the class consisting of copper, zinc,

lead and silver, comprising immersing the article 'in a plating bath containing an aqueous solution. of chromic acid in-part free and in part combined as an alkali salt of chromic acid, the total chromic acid being about 30 to 50 oz. per gallon and the free acid constituting 20'to about 40% of the total and sulphate in amount of about .1" ounce pergallon, and supplying plating current to said article.

. herent chromium upon an article having a metal- '6. A process of plating clear, adherent vchromium upon a metal surface composed preponderantly of copper comprising first treating the surface in a bath having an etching effect upon said. metal the products of which are soluble in said bath, washing said surface, then immersingsaid article in a. plating bath containing an aqueous" solution of chromic acid in part free and in part combined as an alkali salt of chromic acid, the

aiaam total chromic acid being about 80 to 50 as. per gallon and the free acid constituting to about of the total, and sulphate acid catalyst to the amount of not over approximately .4% of the total chromic acid, and supplying plating current to said article.

'1. A process oi! plating clear, adherent chromium upon a brass or copper surface comprising first subjecting such surface to a cathodic treatment in an alkaline cyanide solution, washing with dilute acid to free from alkali, then plating directly thereon a coating of chromium in a plating bath comprising an aqueous solution of chromic acid in part tree and in part combined as an alkali salt of chromic acid, the total chromic acid being about 30 to 0s. per gallon and the free acid constituting 20 to about 40% or ,the total, and sulphate acid catalyst to the gallon of water.

amount of not over approximately .4% of the total chromic acid, and supplying plating current to said surface.

8. The process of making a chromium plating bath for plating clear, adherent chromium comprising mixing the following ingredients in substantially the proportions specified; chromic acid pounds, sodium carbonate 36 pounds, alumina hydrate (commercial) 19 oz., water 8 pounds, said constituents including'sufflcient sulphate to provide a sulphate content 01' approximately .2% of the dry composition resulting from mixing said ingredients in the proportions specifled, and dissolving said mixture of ingredients in water in-the ratio of about three pounds per FRANKH.BE ALL. 

